1. Field of the Invention
The present invention relates to an engine system including a bypass passage that bypasses a throttle valve provided in an air intake passage that supplies air to an engine.
2. Description of the Related Art
US2005/0272320 A1 discloses a vessel propulsion system that drives a propeller with an internal combustion engine. The propulsion system includes a throttle valve disposed in an air intake passage of the engine, a secondary air passage that places a portion of the air intake passage downstream of the throttle valve in communication with the atmosphere, and an idle speed control valve (ISC valve) disposed in the secondary air passage. When the throttle valve is fully closed, the ISC valve is feedback-controlled based on an engine speed to adjust an intake air amount taken in via the secondary air passage. Thus, the engine speed is controlled to be at an idling speed that is set in advance. A dash pot control using the ISC valve is also performed. In the dash pot control, the occurrence of an engine stall upon performing a rapid valve closing operation of the throttle valve is avoided by opening the ISC valve to correct an insufficiency of the intake air amount. Specifically, when an opening degree of the throttle valve is large, a large amount of fuel is supplied to the engine. When the throttle valve is rapidly closed from this state, a proportion of the air amount with respect to the fuel supplied inside the engine becomes insufficient, a mixing ratio of fuel to air degrades rapidly, which leads to engine stall. The engine stall can be avoided by opening the ISC valve to compensate for the insufficiency in the amount of air.